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Old 04-21-2010, 08:04 PM   #1
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Multi Wire Circuit Question


I own a home that has a 200A service panel outside that houses all of the 220V circuits with a 70A double pole breaker supplying a sub-panel in the laundry room. All of the circuits coming out of the sub panel are 12-3 multi-wire circuits.

The house was built in 1983, so I'm not sure what the electrical code was back then, but when mapping out the circuits, I discovered that all of the multi-wire circuits, in my opinion, are on the same phase.

I traced them out and found 0V when measuring across the breakers going to each circuit (cable). I thought that I should be getting 240V if they were indeed out of phase. I also thought that by current NEC the breakers should be double pole or should be mechanically linked so that both 120V legs are off at the same time. Any help would be much appreciated.

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Old 04-21-2010, 08:13 PM   #2
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Do you measure 240 volts anywhere? For example between one breaker of a breaker pair and one breaker of the breaker pair further on down?

You might be able to move the breakers about in the panel so they span fins on the bus bars that are on opposite legs.

Or maybe the 70 amp breaker in the main panel with the feed to the subpanel is not optimally positioned, spanning two fins of the same side of the line up in the main panel.

Yes, the breakers for each 120/240 volt circuit aka multiwire branch circuit down in the subpanel must be a double wide breaker pair (2 pole breaker unit) with the handles tied together.

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Old 04-21-2010, 08:25 PM   #3
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Multi Wire Circuit Question


Can you post a pic?
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Old 04-21-2010, 08:28 PM   #4
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The only place that there is 240V is where it comes into the panel at the top. I always thought that these multi-wire branch circuits would overheat the neutral and cause a fire if placed on the same phase. This appears to be the way it has been since 1983. It doesn't appear that the neutrals are brittle or anything. It would be quite a job to reorganize the panel. There doesn't seem to be a pattern except that they are all on the same phase. It looks like a nice and neat job.
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Old 04-21-2010, 08:42 PM   #5
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I thought that I should be getting 240V if they were indeed out of phase.
The two 120v must be in phase to add to 240v. It's a center-tapped 240v secondary on a transformer.

Figuring out the wiring on a load center by making measurements can be tricky. You don't really want to use an ohmmeter and the breakers cover up the conductor routing.

Last edited by Yoyizit; 04-21-2010 at 08:45 PM.
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Old 04-21-2010, 08:53 PM   #6
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When you say the same phase are you saying that the breakers are on the same side of the panel? Normal panels have the two hot legs interlaced like fingers from both hands, ie every other one is on a opposite leg.
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Old 04-21-2010, 09:11 PM   #7
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The only place that there is 240V is where it comes into the panel at the top. I always thought that these multi-wire branch circuits would overheat the neutral and cause a fire if placed on the same phase. This appears to be the way it has been since 1983. It doesn't appear that the neutrals are brittle or anything. It would be quite a job to reorganize the panel. There doesn't seem to be a pattern except that they are all on the same phase. It looks like a nice and neat job.
Something is not adding up in this picture. If --as you say-- You DO measure 240v. across both HOT legs at the top of the panel, you should be measuring 240v. at some point in the panel. Even if the breakers are not 2-pole or with a (common) attached handle. To the best of my recollection (from a practical point), MWBC were ALWAYS wired either to 2-pole brkrs. or common-trip. Furthermore, even when cartridge fuses were in use, mwbc and 240v. circuits were wired to a common socket, which was pulled (out) as one unit. One possible reason for not reading 240v. across the breakers could be that some lower-end (hate to use the word "cheap".) panels were manufactured (by FPE (RIP) and GE in such way that the bus bars were not intertwined but ran to the center of the panel on either side, and there were only 2 spots for 240v. The rest were on the same leg (120v. or Zero v.) In any case, the electrician/s who wired it did not do a good job (at least of testing after installation.)!
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Old 04-21-2010, 09:43 PM   #8
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Something is not adding up in this picture. If --as you say-- You DO measure 240v. across both HOT legs at the top of the panel, you should be measuring 240v. at some point in the panel. Even if the breakers are not 2-pole or with a (common) attached handle. To the best of my recollection (from a practical point), MWBC were ALWAYS wired either to 2-pole brkrs. or common-trip. Furthermore, even when cartridge fuses were in use, mwbc and 240v. circuits were wired to a common socket, which was pulled (out) as one unit. One possible reason for not reading 240v. across the breakers could be that some lower-end (hate to use the word "cheap".) panels were manufactured (by FPE (RIP) and GE in such way that the bus bars were not intertwined but ran to the center of the panel on either side, and there were only 2 spots for 240v. The rest were on the same leg (120v. or Zero v.) In any case, the electrician/s who wired it did not do a good job (at least of testing after installation.)!
I can measure 240 in the panel if I connect my test leads to breakers that are out of phase. The main thing I'm worried about is any two single pole breakers that connect to the red and black in each 12-3 cable are (1.) Not next to each other (out of phase). (2) They don't read 240V when I measure the voltage by touching one lead to one screw on a breaker and the other lead to the other screw on the other breaker. This would be measuring the voltage in series and should read 240V if out of phase, 0V if in the same phase. I'm not an electrician nor an electrical engineer, so maybe I don't understand electrical phases like I need to. I don't know if this maters or not but I tested the two wires on the double pole breaker on the main service panel outside and that read 240V on the black and red wire. Is it possible that the two hot wires feedin the sub panel are actually on the same phase even though they're next to each other?
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Old 04-21-2010, 09:44 PM   #9
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Watch out for arc-flash; it's not an obvious danger to non-electricians.
And a recent issue of EC & M had an article about a guy who fatally burned while measuring a panel with a meter that was not designed to handle the power levels present, although the meter manual implied that it was.
The last thing he said was, "Something's not right, here."

Your load center can probably supply 11,000 amps, which is what I measured mine at.

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Old 04-21-2010, 09:48 PM   #10
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That doesn't sound good. Explain.
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Old 04-21-2010, 09:51 PM   #11
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That doesn't sound good. Explain.
http://en.wikipedia.org/wiki/Arc_flash
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Old 04-21-2010, 09:54 PM   #12
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I can measure 240 in the panel if I connect my test leads to breakers that are out of phase. The main thing I'm worried about is any two single pole breakers that connect to the red and black in each 12-3 cable are (1.) Not next to each other (out of phase). (2) They don't read 240V when I measure the voltage by touching one lead to one screw on a breaker and the other lead to the other screw on the other breaker. This would be measuring the voltage in series and should read 240V if out of phase, 0V if in the same phase. I'm not an electrician nor an electrical engineer, so maybe I don't understand electrical phases like I need to. I don't know if this maters or not but I tested the two wires on the double pole breaker on the main service panel outside and that read 240V on the black and red wire. Is it possible that the two hot wires feedin the sub panel are actually on the same phase even though they're next to each other?
If you have 240v. in the subpanel, at any point, it means that the 2-pole breaker in the main panel IS situated correctly. And is feeding the right voltage to sub panel!
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Old 04-21-2010, 10:14 PM   #13
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BPC, What are the conductor colors in what you call your 12-3 cable?
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Old 04-21-2010, 10:17 PM   #14
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I would like to see a picture. If the OP could show where the voltage measurements were taken that would be even better.

Perhaps someone used tandems?
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Old 04-21-2010, 10:33 PM   #15
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Having trouble uploading a picture because the file is too large. I'll try to get it posted soon. I'm positive I'm measuing correctly, although that last post about the guy gettin burned has turned me off on testing live circuits. I guess you can't just take it for granted that the meter that you're using will do the job. I used a Fluke 88 to do the test which is a pretty good quality meter, but from now on I'll be extra careful.

Colors on the wire in the 12-3 are red, black, white and bare copper.

Someone in an earlier reply suggested that I just move one of the breakers on each branch untill I read 240V. I really didn't want to do this and am thinking of just calling an electrician. What I don't understand is how it got by for the last 25 years without a problem?


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